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On the basis of the data collected on a pelagic longline vessel from November 18, 2012 through March 31, 2013 in the fishing area of the Indian Ocean (2°47′N~8°13′S,62°18′E~ 67°49′E), the capture depth of the dominant bycatch species and the relationship between their catch rates and the sea surface temperature were analyzed. The results showed that (1) blue shark (Prionace glauca) mainly inhabited the water layer of 80~160m, the water layer with the highest catch rate was 120~160m, followed by 80~120m, the catch rate of remaining water layers was low; (2) swordfish (Xiphias gladius) mainly inhabited the water layer of 80~200m, the catch rate of this water layer increased at first then decreased, the catch rate in the water layer of 120~160m was the highest and much higher than that of other water layers; (3) blue marlin (Makaira nigricans) was mainly caught in the water layer of 80~200m, the catch rate of this water layer was high, and the catch rate peaked in the water layer of 160~200m. The catch rate in the water layer of 200~280m was low, and decreased with depth; (4)striped marlin (Tetrapturus audax) was caught in the water layer of 80~200m, no striped marlin was caught in other water layer, the catch rates decreased with depth; (5) crocodile shark was caught in the water layer of 200~320m, no crocodile shark was caught in other water layer, the catch rates increased with depth; (6)the catch rates of blue shark increased with the increasing of the sea surface temperature, peaked at 30.1~30.5°C; the catch rates of swordfish and blue marlin peaked at 29.6~30°C; the catch rates of striped marlin were high at 29.6~31°C and peaked at 30.1~30.5°C; the catch rates of crocodile shark peaked at 30.6~31°C. This study suggested that the depth of the pelagic longline hook should be deployed deeper than 160m and shallower than 280m or avoid operation in the area where the sea surface temperature is higher than 29.6°C to reduce the bycatch of blue shark, swordfish, blue marlin and striped marlin.
